Ferrite power limiter comprising synchronously tuned, resonant cavities

ABSTRACT

A section of microwave transmission line is made in the form of three contiguous, in-line, synchronously-tuned, resonant cavities. The end of each cavity is terminated by a symmetrical inductive iris and the spacing between irises is approximately a half wavelgnth at the center frequency of the selected passband. In each cavity there are supported alternate rods of ferrite and nonmagnetic dielectric material all of the same dimensions and of substantially the same dielectric constant. An adjustable magnetic field is provided for the ferrite. The limiting threshold is greatly lowered because of the combined effects of increased RF magnetic field intensity in the cavities compared to that in the transmission line and the nonmagnetic separator rods that direct more of the magnetic flux through the ferrite.

" United States Patent [1 1 [111 3,906,404

Dixon,Jr. Sept. 16, 1975 [5 FERRITE POWER LIMITER COMPRISING PrimaryExaminerPaul L. Gensler SYNCHRONOUSLY TUNED, RESONANT Attorney, Agent,or Firm-Nathan Edelberg; Robert P. CAVITIES Gibson; Arthur L. Bowers[75] Inventor: Samuel Dixon, Jr., Neptune, NJ. [57] ABSTRACT Assignee:The United states of America as A section of microwave transmission lineis made in represented y the Secretary of the the form of threecontiguous, in-line, synchronouslyys Washingtonv tuned, resonantcavities. The end of each cavity is ter- [22} Filed: Feb. 25, 1974minated by a symmetrical inductive iris and the spacing between irisesis approximately a half wavelgnth at PP 445,737 the center frequency ofthe selected passband. In each cavity there are supported alternate rodsof ferrite and 52 us. Cl. 333/17; 333/242 nfmmagneic dielectric materialthe Same dimem [51] In. CL u Holp U00; 01p 1/22 sons and ofsubstantially the same dielectric constant. 581 Field of Search 333/17,73 w, 24.2 An adjustable magnetic field is Pmvided for the rite. Thelimiting threshold is greatly lowered because [56] References Cited ofthe combined effects of increased RF magnetic field intensity in thecavities compared to that in the UNITED STATES PATENTS transmission lineand the nonmagnetic separator rods 3,131,366 4/1964 Dixon, .11. 333/242that direct more of the magnetic flux through the 3,500,256 3/1970Carter et a] rite 3,629,735 12/1971 Carter et a] 333/17 1 Claim, 4Drawing Figures 18 FERRITE m |R| S w '6 32 3O DIELECTRIC FERRITE a O oPmmmstmma 3,906,404

SHEET 1 pg 2 nus DIELECTRIC FERRITE 6 III I PATENTEUSEP 1 6191s 3 36,404

SHEET 2 BF 2 FIG. 4

INSERTION LOSS (db) POWER (WATTS) THRESHOLD FREQUENCY (GHz) BACKGROUNDOF THE INVENTION Most high frequency power limiting devices now arefabricated around a nonlinear element, generally a ferrite. Ferriteshave proven to be more reliable as power limiters than predecessor gasTR tubes. One disadvantage' of ferrite power limiters has been narrowbandwidth, which has been nominally 3% at K, band (12-18 gigahertz).Narrow bandwidth has restricted ferrite limiter usefulness to radarapplications. Another disadvantage of ferrite power limiters has beenthreshold levels of at least 20 watts, too high for broadband wideopenreceivers.

Ferrite has three nonlinear physical mechanisms that may be exploitedfor power limiting. These mechanisms are known as premature decline ofthe main resonance, subsidiary resonance, and coincidence of the mainresonance with the subsidiary resonance. The first and third of thesemechanisms are applicable to devices that are too narrow-band and havethresholds that are too high for a broadband wide-open receiver.Generally, ferrite power limiter devices have been designed to utilizethe subsidiary resonance mechanismbecause it offers broader frequencyrange and sharper frequency selectivity. These characteristics ofsubsidiary resonance are particularly suited for electronic warfarereceivers and troposcatter communication receivers that operate in Kband. In electronic warfare systems, the frequency selective propertymakes it possible to monitor a plurality of signals within the passbandof the receiver and if one or more monitored signals exceed the limitingthreshold and others do not, the signals that exceed the threshold arelimited to threshold level and the other signals are passed unaffected.In troposcatter communication systems, the frequency selectivepropertyallows more efficient allocation of the frequency spectrum byreducing the frequency separation of transmitter and receiver.

Ferrite used for subsidiary resonance is crystalline, ferrimagnetic andnonconductive. Each molecule of crystal has a magnetic moment. A DCmagnetic field is established through the crystal in a direction suchthat the magnetic moments are aligned with the applied magnetic field.Microwave energy traversing the ferrite material cause the magneticmoments to precess relative to the magnetic field direction at a ratedetermined by constants of the material. Output is directly proportionalto input up to critical threshold power. At critical threshold power,spin waves are generated in the material and more of the incidentmicrowave energy is absorbed. This nonlinear action limits microwaveoutput power as microwave input power at any frequency within thepassband increases from the threshold level. When a power limiteroperates in subsidiary resonance and two or more time-coincident signalsof different frequencies within the passband are transmitted to thepower limiter and one signal exceeds the threshold, the limiter operatesto limit the one signal, while passing the other signals withsubstantially no attenuation. It is unnecessary to separate out signalsthat exceed the threshold for transmission through the limiter.

There have been two problem areas in the design of a subsidiaryresonance limiter. First, the relaxation time of the ferrite materialpermits pass-through of a substantially full amplitude spike of thesteep leading edge of an input pulse. Second, the limiting threshold ofsubsidiary resonance ferrite limiters has been too high for the crystalat the input end of a receiver. Since relaxation time is a basicmaterial parameter that cannot be decreased, this problem is minimizedby using 'the best ferrite material available. Single crystal YIG(yttrium iron garnet) is the best material in the range from S-band to X-band. Above X-band, at about 16 gigahertz there is a breakout frequencyat which single crystal lithium ferrite with a higher saturationmagnetization has a lower threshold that single crystal YIG. In mostreceivers of electronic warfare systems and troposcatter communicationsystems, the minimal amplitude spike transmitted by the best ferrite maybe tolerated because it generally does not have sufficient energy tocause crystal burnout-The main problem is the limiting threshold whichis too high for broadband wide-open receivers.

An object of this invention is to provide an improved microwavetransmission line with ferrite power limiter for use over a broad bandand that has a substantially lower limiting threshold than prior artarrangements.

A further object is to provide a broadband microwave energy transmissionline with a threshold on the order of 0.5-2.0 watts, flat response andnegligible spike leakage over a wide passband that is approximately15-17 gigahertz and thathas a relatively; ide dynamic range, e.g. atleast 20 db. I i A SUMMARY OF THE INVENTION A substantial improvement insubsidiary resonance ferrite limitersin terms of reduced threshold powerand increased frequency range of operation is achieved by employingsynchronously tuned, multiple resonance cavities, in-line in a waveguidetransmission line. A ferrite limiter structure is in at least one of thecavities or in all of the cavities. Because there i'sintensification ofRF magnetic field'in a resonancecavity compared to that in thetransmission line, the interaction of cavity and ferrite operates tolower the limiting threshold. The ferrite structure is of alternate rodsof ferrite and of dielectric material of equal dielectric constant andthe dielectric provides magnetic isolation between ferrite rods.

DESCRIPTION OF A PREFERRED EMBODIMENT FIG. 1 is a view in perspective ofa short length of a microwave transmission line that includes theinvention as part of the transmission line,

FIG. 2 is an exploded view of an embodiment of the invention shown inFIG. 1 on a larger scale, and not including the coupling flanges of FIG.1,

FIG. 3 is an end view of the assembled embodiment of the invention shownin FIG. 2, on a smaller scale than in FIG. '2, and

FIG. 4 is a graphical showing of passband, insertion loss and powerlimiting character of the described embodiment.

A broadband limiter structure 10, shown in outline in FIG. 1, isseries-connected in microwave transmission line 12. Though thetransmission line 12 is shown as rectangular waveguide, the rectangulargeometry is not essential in this invention and is not intended as alimitation. Conventional coupling flanges 14 affix the limiter structure10 in series in the microwave transmission line. A magnetic field vectorI-I represents DC magnetic field generated by a conventional selectivelyadjustable means for directing a steady magnetic field through thestructure in the direction of the arrow or in the reverse direction andof selected intensity. The magnetic field may be uniform or taperedalong the length of the limiter structure 10.

The embodiment of broadband limiter 10 shown in FIG. 2 includes a pairof identical complementary channel-like housing members 16, 18 with.sets of aligned bolt holes such as 24, 26. A flatbar 28 nests in thebight of each channel member 16, 18. The bars 28 are of the same lengthas members16, 18 and each bar 28 and each member 16 18 are formed withholes for screw fastening the bars 28 inplace in the members 16, 18. Thedistance between the bars 28. and the width of the channels of members16 and 18 shown in FIG. 3 correspond to the height and width dimensionsof the interior of waveguide 12. The limiter structure forms a bridgingsection in the interrupted length of waveguide 12. Three resonantcavities are defined by four matched pairs of iris inserts 30 removablyfastened to bars 28 byscrews not shown in FIG. 2. The iris inserts areperpendicular to bars 28 and extend across the full width of the bars.The cavities are coupled by the symmetrical inductive spacings defined.by the irises. In each cavity, there are supported at least two ferriterods 3 2 separated by dielectric rods 34 all of the same dimensions,cemented across one or both of the bars 28 iri the E-field direction. Asuitable material for dielectric 34 is barium titanate. The dielectricis nonmagnetic and serves the purpose of causing the magnetic fieldintensity to be greater in the adjacent ferrite than it would beotherwise. By the use of two ferrites, i.e. a combination of singlecrystal YIG and single crystal lithium ferrite rods along the limiter,there was achieved a threshold of 0.75 watts at 16.0 GHz to 2.0 watts atl7.0.CvHz with insertion loss of 0.9 db. Initial design is directed to,the center frequency of the desired passband as resonant frequency. Thedimensions of the waveguide structure 10 are derived empirically instages to approach the desired limiter characteristic. An initial set ofdimensions are established taking into account the number of cavitiesdesired. The ferrite 32 and dielectric 34 are installed parallel to theE-field direction. Tests are made using iris inserts of severaldifferent dimensions. The design is improved in this manner until it isoptimized in terms of desired bandwidth and insertion loss. Cavitydimensions may be changed during the process. Increasing the number ofresonant cavities steepens the skirts of the passband. This inventioncontemplates any number of cavities including a single cavity.

The cross section of the ferrite rod is madesmall in comparison to itslength for more efficient limiting. If the volume of ferrite material istoo small the dynamic range is inadequate. The volume of material isincreased by increasing the number of ferrite elements. Contiguousferrite elements in each cavity are separated by dielectric material ofthe same dimension and dielectric constant as the ferrite. Only twoferrite rods are shown in each cavity. They may be made thinner thanshown and their number may be increased.

When using single crystal YIG rods as the ferrite, addition of externalmagnetic field downshifts the passband. However, the leading edge skirtof the passband is shifted upward by increasing the magnetic field intensity. Therefore the bandwidth can be stretched by using a taperedmagnetic field. Upon comparing the rev 17.0 GH: and the response forlithium ferrite was approximately 2.0 watts from-16.0 to 17.0 Gl-Iz.Magnetic .field intensity was on the order of 2000-3000 gauss.

Between 16 GHz and 17 G112 a combination of single crystal YIG rods andsingle crystal lithium ferrite as the ferrite rodsalong the cavities asshown in FIG. 2 manifests better, characteristics than either materialalone.

The. synchronously-tuned multiple cavity resonator structure acts as ahigh Q filter over a very wide bandwidth because symmetrically inductivecoupling elements are essentially perfect impedance inverters orquarterwave transformers. Because the quarter Wave quality of theimpedance inverter is broadband, design accuracy is good over a Widerange of frequency. The magnetic field H may be tapered along thelongitudinal dimension of the multiple cavity structure in order thatthe ferrite be active over a wide passband; signals at widely separatedfrequencies within the passband are subject to limiting without changingthe preadjusted external magnetic field.

FIG. 4 is a typical example of the relationship among passband,insertion loss, and threshold of an embodiment of this invention asshown in FIG. 2. Insertion loss on the order of one db across a 2+gigahertz passband is readily achieved. The limiting threshold is shownby the dashed line. There are shown two input signals f, and f of widelyseparated frequencies; only f exceeds the threshold. The power level ofsignal f is reduced to the threshold level while the signal f traversesthe limiter with negligible attenuation. Two signals f and f are notshown in any limiting sense. Theremay be one or there may be severalsignals. In any one time period signals occur at the same frequencies.Subsequently, signals at one frequency cease and signals at anotherfrequency commence. During any given time interval there may be nosignals or there may be signals at several spaced frequencies in thepassband.

What is claimed is:

l. A broadband ferrite limiter for connection between two rectangularwaveguide transmission line sections for the frequency range 16-17 Gl-Izcomprising a pair of complementary channel-like housing memberscontiguous with each other so as to form a rectangular waveguide, a flatbar secured in the bight of each channeI-like housing member, the widthsof the bars and of the channels in thehousing members being the same,

several symmetrical iris means secured along the lengths of the bars todefine a plurality of resonant cavities between iris means in the 16-17(31-12 frequency range, at least two rectangular ferrite rods affixednormal to one face of one of said bars between each pair of successiveiris means, every two successive ferrite rods between successive irismeans sandwiching and separated by a nonmagnetic dielectric rod, theferrite rods including single crystal YIG rods and single crystallithium ferrite rods, all of said rods having substantially the samedimensions and the same dielectric constant, whereby in the presence ofa dc magnetic field of 2000-3000 gauss the limiting threshold is lowbecause of the combined effects of RF magnetic field intensity in thecavities and the nonmagnetic separator rods that direct more of themagnetic flux through the ferrite and whereby any propagatedsignal'within the frequency 3,906,404 6 range that exceeds criticalpower level is limited to critthat exceed critical power level andsignals that do not ical power level and any propagated signal withinthe exceed critical power level but differing in frequency frequencyrange that does not exceed the critical power are substantially timecoincident. level is substantially unattenuated even when signais

1. A BROADBAND FERRITE LIMITER FOR CONNECTION BETWEEN TWO RECTANGULARWAVEGUIDE TRANSMISSION LINE SECTIONS FOR THE FREQUENCY RANGE 16-17 GHZCOMPRISING A PAIR OF COMPLEMENTARY CHANNEL-LIKE HOUSING MEMBERSCONTIGUOUS WITH EACH OTHER SO AS TO FORM A RECTANGULAR WAVEGUIDE, A FLATBAR SECURED IN THE BIGHT OF EACH CHANNEL-LIKE HOUSING MEMBER, THE WIDTHSOF THE BARS AND OF THE CHANNELS IN THE HOUSING MEMBERS BEING THE SAME,SEVERAL SYMMETRICAL IRIS MEANS SECURED ALONG THE LENGTHS OF THE BARS TODEFINE A PLURALITY OF RESONANT CAVITIES BETWEEN IRIS MEANS IN THE 16-17GHZ FREQUENCY RANGE, AT LEAST TWO RECTANGULAR FERRITE RODS AFFIXEDNORMAL TO ONE FACE OF ONE OF SAID BARS BETWEEN EACH PAIR OF SUCCESSIVEIRIS MEANS, EVERY TWO SUCCESSIVE FEREITE RODS BETWEEN SUCCESSIVE IRISMEANS SANDWICHING AND SEPARATED BY A NONMAGNETIC DIELECTRIC ROD, THEFERRITE TODS INCLUDING SINGLE CRYSTAL YIG RODS AND SINGLE CRYSTALLITHIUM FERRITE RODS, ALL OF SAID RODS HAVING SUBSTANTIALLY